Low alloy, high strength, age hardenable steel

ABSTRACT

Low alloy, high strength, age hardenable steel, especially adapted for bar product applications, consisting essentially in percentage by weight of about: up to 0.06% maximum of carbon, from more than 1% up to 8% chromium, 0.25 to 3.5% nickel, 1 to 4% copper, 0.01 to 0.5% niobium, 0.2 to 2% manganese, 0.01 to 1% silicon, up to 1.5% molybdenum, up to 1.2% aluminum, up to 0.5% titanium, up to 0.1% boron, up to 0.035% phosphorous, up to 0.040% sulphur, balance iron except for impurities within commercial tolerances, characterized as hot rolled and air cooled or as thereafter solution treated at about 1,600*-2,200*F and thence air or more rapidly cooled, by room temperature hardness of about 30 Rockwell &#39;&#39;&#39;&#39;C&#39;&#39;&#39;&#39; and under, suitable for forming and machining, and as thereafter aged at about 700*-1,100*F, by room temperature properties as follows: tensile strengths of about 160-180 ksi, yield strengths of about 135-145 ksi, tensile elongations of about 17-20%, Rockwell &#39;&#39;&#39;&#39;C&#39;&#39;&#39;&#39; hardnesses of about 35-40, and Charpy impact energies of about 15-30 ft-lbs. and higher depending on the heat treatment.

United States Patent 11 1 Waid et al.

[451 Aug. 12, 1975 LOW ALLOY, HIGH STRENGTH, AGE

HARDENABLE STEEL [75] Inventors: George M. Waid, Williamsfield; Robert T. Ault, Shaker Heights, both of Ohio [73] Assignee: Republic Steel Corporation,

Cleveland, Ohio 22 Filed: Dec. 13,1973

21 Appl. No: 424,552

[52] US. Cl. 148/36; 75/125; 75/128 G; 148/12 F [51] Int. Cl. C221: 39/26 [58] Field of Search. ..75/128 G, 125, 128 A, 75/128 W. 128 T, 128 F; 148/12 F, 36

[56] References Cited UNITED STATES PATENTS 3,132,025 5/1964 Hurley 75/124 3,262,777 6/1966 Sabowski 75/128 G X 3,347,663 10/1967 Bieber i 75/125 X 3,472,706 10/1969 Kattus et a]... 75/125 X 3,475,164 10/1969 Hydrean 75/128 G X 3,661,565 5/1972 Harvey 75/125 3,692,514 9/1972 Hydrean 75/124 3,719,476 3/1973 Tanczyn 75/125 3,824,096 7/1974 Asada et al. 75/125 X Primary ExaminerL. Dewayne Rutledge Assistant ExaminerArthur J. Steiner Attorney, Agent, or FirmRaymond J. McElhannon 5 7 ABSTRACT Low alloy, high strength, age hardenable steel, especially adapted for bar product applications, consisting essentially in percentage by weight of about: up to 0.06% maximum of carbon, from more than 1% up to 8% chromium, 0.25 to 3.5% nickel, l to 4% copper, 0.01 to 0.5% niobium, 0.2 to 2% manganese, 0.01 to 1% silicon, up to 1.5% molybdenum, up to 1.2% aluminum, up to 0.5% titanium, up to 0.1% boron, up to 0.035% phosphorous, up to 0.040% sulphur, balance iron except for impurities within commercial tolerances, characterized as hot rolled and air cooled or as thereafter solution treated at about 1,6002,200F and thence air or more rapidly cooled, by room temperature hardness of about 30 Rockwell C and under, suitable for forming and machining, and as thereafter aged at about 7001,100F, by room temperature properties as follows: tensile strengths of about 160-180 ksi, yield strengths of about 135145 ksi, tensile elongations of about 17-20%, Rockwell C hardnesses of about 35-40, and Charpy impact energies of about 1530 ft-lbs. and higher depending on the heat treatment.

5 Claims, 1 Drawing Figure LOW ALLOY, HIGH STRENGTH, AGE ARDEN ABLE STEEL This invention pertains to a low carbon, low alloy, age hardenable steel, especially adapted for bar product applications, of high strength, ductility and toughness in the age hardened condition, but of relatively low hardness as solution treated for ease of forming into articles and machining.

The principal required alloying elements of the steel ar nickel, chromium and copper together with small amounts of niobium. Silicon and maganense are also included in amounts which occur in the melting of normal fine grained aluminum killed steels. The steels may also contain small amounts of one or more of Mo, Ti,

Al and B as optional additions.

The invention provides a low alloy (low alloying content with respect to such steels as maraging or stainless), high strength steel which is age hardenable, which can be machined or fabricated to close tolerances easily while in a relatively soft condition, after hot working and air cooling (or more rapid quench), or after hot working, air cooling, solution treating and either air cooling or oil or water quenching, then strengthened by an aging treatment to obtain a desired strength level with little distortion andno need for a 1 subsequent finish machining operations. The steel of the invention involves both a specific chemistry range and a specific processing sequence. The addition of copper provides the principal precipitating phase which is present in the aged steel as e-copper. This precipitate accounts for most of the strengthening that occurs during the aging treatment. The processing sequence consists of hot working the steel into bar product forms, then air cooling.

Preferably, the material should be in a soft condition after air cooling and ready for machining or fabrication; however, if this is not the case, an intermediate heat treatment can be employed. This heat treatment consists of heating the steel in the temperature range of about 1,600 to 2,200F, thence air cooling or quenching in either oil or water. This treatment imparts to the steel a soft, machinable structure. After the machining process is complete, merely heating the steel in the temperature range of about 700 to 1,100F for various lengths of time (usually 1 to 6 hours), depending on the aging temperature used, will yield a hardened workpiece with ultimate tensile strengths exceeding 160 ksi.

The broad, preferred and optimum alloy ranges for the steel of the invention are:

Weight Percent Element Broad Preferred Optimum C 0.06 max. 0.05 max 0.04 max. Mn 0.2-2.0 0.3-0.8 0.4-0.8

Si 0.0l-l.0 0.2-0.5 0.2-0.35 Ni 025-35 0.5-1.5 0.5-1.5 Cr l.0-8.0 1.5-5.0 1.5-4.5 Cu 1.0-4.0 1.5-3.5 1.5-3.5 Nb 0.0l().5 0.02-0.45 0.0Z-0.3 Mo 0-1.5 00.25 0-0.12 Al 0-l.2 0-0.25 0-0.l5 Ti 0-0.5 0-0.25 0-025 B 0-0. I 00.005 0-0005 P 0.035 max. 0.025 max. 0.020 max. S 0.040 max. 0.030 max. 0.020 max. Hal. Fe Fe Fe UTS YS RA El Charpy Hardness Ksi Ksi Ft-lbs. Rc

175 140 60 18 25 min. 35-40 Hardness as solution treated and quenched is typically about 25-30 Re.

The alloy element contents are designed to develop' a steel which can be easily machined or fabricated to close tolerances while in a relatively soft condition, after hot working and air cooling (or more rapid quench), or after hot working, air cooling, solution treating and either air cooling or water or oil quenching. Subsequently, an aging treatment is used to obtain the desired strength level. Specifically, the major alloying elements are added for the following reasons:

1. Copper is the principal strengthening element in this alloy system. During the aging process it precipitates as e-copper particles-causing a substantial increase in strength.

2. Nickel is added for two purposes. The first is to prevent hot shortness in this copper containing steel and the second is to improve the impact toughness properties;

3. Carbon is restricted to a maximum of 0.06 weight percent because of its deleterious effects on impact toughness. Carbon also raises the base hardness of this alloy system causing machining of articles made from the proposed alloy steel to be more difficult.

4. Columbium (niobium) is added both for its grain 4Q refining effects during hot rolling, thus increasing both strength and impact toughness properties, and the precipitation of niobium carbides andd nitrides during the aging treatment which causes an increase in strength properties.

5. Chromium is added to achieve sufficient aircooled hardenability in bars up to 2 inch round. it has been found that chromium improves impact toughness properties of this alloy, up to approximately four (4.0) weight percent thereof.

50 6. Molybdenum is essentially a residual element,

however, higher amounts of this element, up to 1.5 weight percent may be added to reduce the susceptibility of this alloy steel to temper embrittlement.

7. The additions of manganese and silicon are at levels which occur in the melting of normal fine grained steels.

8. Additions of aluminum, if any is added, is either for the purpose of reducing grain size during hot rolling, thus complementing the niobium and/or columbium additions, or for deoxidation purposes if the steel of this invention is to be aluminum killed.

The following Tables I-lll inc., give room temperature Rockwell C (Rc) hardness and Charpy impact (CVN) energy in ft--lbs. for test results on steels of the tabulated compositions in the various heat treated conditions shown therein.

Processing and Heat Trentment TABLE III As-Rolled Composition Mailed (Ind Aged D.. E .-E EP. fli fl "0725 Same Chemistry 00 Above 24 9 8 29 41.5

=C726 Same Chemistry as Abpve 26 52 32. 10.7

C727 Same Chcmistry as Above 24.5 50 31 8.5

C728 flame Chemistry I! Above H 27 18 34 4.5

*C729 .016 .97 .25 1.0 .06 1.0 v 2.25 .088 20 151. 21. **C729 Same Chemistry ea Above 23 3 28 17.5

C730 .017 1.00 I .26 1.5 I .063 1.0 2.2 .08 20:5 153 24.5 143 "C730 Same Chemistry as Above 24 53 30 13 "C731 Same Chemistry as Above 25.5 64 1 32 11 A.C. and Aged R CVN Remarks 2100 F Aust.

Temp

Aust Temp Aust. Temp Aust. Temp Aust, Temp Aust. Temp Aust Temp Aust- Temp Aust. Temp Ausc.

Temp

Au st. Temp Aust. Temp Aust. Temp Aust, Temp Aust. Temp Aust. Temp AusL. v

Temp

Aust. Temp Aust. Temp Aus 1:.

Temp

Aust.

Temp

Aust. Temp Aust. Temp Aust. Temp Aust. Temp Aust. Temp Aust. Temp {TABLE III continued! Processing and Heat Treatment As-Rolled A.C. Composition I AS'ROllGd and Aged 11.9 W.Q. & Agej A,C, and Aged Egg C l i n ii Cr Mo ll i Q C R CVN i CVN l3 CVN i B M Remarks *C732 .033 .99 .28 2.6 .063 .98 2.2 .079 105 -29.5.' 78 26 3 0.5 6 24 32 7.2 2100 F Aust.

. I Temp 22.5 30.5 v 57 21.5 25.5 117 1700 r Ausc Temp (I7 2 Same Chemistry as Above 26 43 "32.5 11.5 25.5 36.5 0.5 25 32.5 7.5 2100 F Aust.

' Temp 26.5 33 48 22 26 92 1700 P Aust- Temp Aged at 900 F for two hours.

* 1850 F soaking temperature. 2250 F soaking temperature.

CVN Charpy v-nctch impact specimens tested at +70 F.

Referring to the data of these tables it will be seen that the steels containing less than applicants lower limit of more than 1.0% chromium do not possess the The following Table IV gives the results of tensile tests on representative steels according to the invention and of the compositions shown in the table.

TABLE IV MECHANICAL PROPERTY DATA ON AGE HARDENABLE BAR PRODUCT ALLOY FROM 1%" ROUND BARS ROLLED ON THE MASSILLON l2 BAR MlLL Heat Soaking Y.S. U.T.S. RA. El.( 2 CVN Hardness No. Temp F ksi ksi /Z '7: ft-lhs. Rc Condition C860 2150 I05 l4) (:7 lo 55 -32 As-Rolled 138 175 56 l8 [2 38-39 As-Rolled and Aged 139 160 66 l 43 -36 Austenitizcd,

Air Cooled and Aged C861 2150 lol 14) 03 lo 32 30-3l As-Rolled 135 I75 47 l7 37-38 As-Rolled and Aged I42 l65 63 l8 3] 3537 Austenitized.

Air Cooled and Aged C933 1900 I09 [56 65 15 32-33 As-Rolled I46 177 (it) 18 26 38-40 As-Rolled and Aged 144 l 77 l 8 29 36-37 Austenitized.

Air Cooled and Aged Composition Heat Not C Mn Si P S Ni Cr Mo Cu Nb Al C860 .03 .63 .25 .008 .009 1.0 3.97 .07 1.95 .l() .0l C861 .03 .63 .23 .015 .020 1.0 4.0l .07 2.l5 .095 .0l C933 .03 .70 .2] .012 .015 l.() 4.00 .06 2.15 .l() .02

Aged '00 F. 2 hrs.

Ansicniti/ed I700 F. l hr. A.(.. :igcd 100 F, 2 hrs.

combination of high hardness and impact strength of those falling within applicants composition range as to chromium. This is further illustrated by the accompanying drawing comprising a graphical plot of chromium content versus hardness and Charpy impact energy, for steels C607, C608 and C609 of the above tables under the heated treated conditions shown in the drawing. The reason for this beneficial effect of chromium is not completely understood, but a reasonable explanation is that the increasing chromium content changes the microstructure of the steel from a mixture of proeutectoid ferrite and low carbon martensite at low chromium levels, to an all low carbon martensite structure at high chromium levels.

The steel of the invention may be produced by melting in the electric furnace. The alloy content provides the air cooled hardenability shown in Table llll inc., in sizes'up to about 2 inch rounds. The product is air cooled from the bar mill and then processed through bar finishing operations to develop commercial straightness and any required surface removal. Components may then be finished machined from the bar stock by the customer and simply aged to achieve strength properties Alternatively, the customer may .austenitize and air cool or oil or water quench before machining. The alloy does not suffer from either machinability or distortion problems upon heat treating as above.

An optimum specific weight percent composition is:

C Mn Si P S Ni Cr Cu 0,03 0.05 0.25 om 0.015 1.0 4.0 2.15

The invention provides in a preferred embodiment, low alloy, high strength, hot rolled bar stock from the mill-(hot rolled at about 2,250-1,850F), primarily intended for steel parts produced by machining a bar of the steel or forging to final part size and tolerance, and thence hardening by aging as above. Further heat treatment and machining to a final size and tolerance is not required.

Insofar as is known to the applicant, the prior art has not developed a steel of the above composition and properties. A unique property of the steel of this invention is that the above mentioned mechanical properties are achieved by a simple aging treatment of the finished 'machined part which requires no subsequent finish machining operations after hardening. In contrast with previously known steels, similar properties are achieved only be producing a roughly machined part from a carbon strenghtened steel, which is then austenitized, oil quenched and tempered to the desired hardness or strength level, which involves some degree of distortion, and when then requires a more difficult finish machining operation of the thus hardened part.

The steel of the invention finds utility, among others, for industrial gearing other than automotive, shafting for pumps, track shoes for off highway equipment, such as caterpiller tractors, power take-off units for farm implements, and heavy machinery.

What is claimed is:

1. Low alloy, high strength, age hardenable, alloy steel consisting essentially in percentage by weight of about: 0.05% C max., 0.30.8% Mn, 0.2-0.5% Si, 0.5-1.5% Ni, 1.5-5.0% Cr, 1.5-3.5% Cu, 0.02-0.45% Nb, O0.25% each of Mo, Al and Ti, -0.005% B, 0.015% P max., 0.02% S max., balance iron except for impurities within commercial tolerances, characterized as hot rolled at about 2,250-l ,850F and air cooled, and also as thereafter solution treated at about l,6002,200F and cooled at least as rapidly as air cool, by a Rockwell C" hardness of about 30 and under, suitable for forming and machining, and as thereafter aged at about 7001,100F by room temperature tensile properties as follows, tensile and yield strengths Bal. Fc

' O.5-1.5% Ni, l.5-4.5% Cr, l.5-3.5% Cu, 0.020.3%

Nb, 00.l2% Mo, 0-0. 15% Al, O0.25% Ti, 0-0.005% B, 0.015% P max., 0.02% S max., balance iron except for impurities within commercial tolerances, characterized as hot rolled at about 2,250-l ,850F and air cooled, and also as thereafter solution treated at about 1,600-2,200F and cooled at least as rapidly as air cool, by a Rockwell C hardness of about 30 and under, suitable for forming and machining, and as thereafter aged at about 700-1,1 10F by room temperature tensile properties as follows, tensile and yield strengths of about 160-180 and -145 ksi, respectively, tensile elongations of about 17-20%, Charpy impact energies of about 15-30 ft-lbs. and higher, and Rockwell C" hardnesses of about 35-40.

3. A wrought article made of a steel according to claim 1 in the condition as hot rolled and air cooled and characterized by a hardness of about 30 Rockwell C and under, adapting the same for forming and machining, and therafter age hardening.

4. A wrought article made of a steel according to claim 1 in the condition as solution treated and characterized by a hardness of about 30 Rockwell C and under, adapting the same for forming and machinine, and thereafter age hardening.

5. A wrought article composed of an age hardened steel according to claim 1 having room temperature, 

1. LOW ALLOY, HIGH STRENGTH AGE HARDENABLE, ALLOY STEEL CONSISTING ESSENTIALLY IN PERCENTAGE BY WEIGHT OF ABOUT: 0.05% C MAX., 0.3-0.8% MN, 0.2-05% SI, 0.5-1.5% NI, 1.5-5.0% CR, 1.5-3.5% CU, 0.02-0945% NB, 0-0.25% EACH OF MO, AI AND TI, 0-0.005% B, 0.015% P MAX., 0.02% S MAX., BALANCE ION EXPECT FOR IMPURITIES WITHIN COMMERCIAL TOLERANCES, CHARACTERIZED AS HOT ROLLED ABOUT 2,250*-1,850*F AND AIR COOLED, AND ALSO AS THEREAFTER SOLUTION TREATED AT ABOUT 1,600*-2,200*F AND COOLED AT LEAST AS RAPIDLY AS AIR COOL, BY A ROCKWELL "C" HARDNESS OF ABOUT 30 AND UNDER, SUITABLE FOR FORMING AND MACHINING, AND AS THEREAFTER AGED AT ABOUT 700*-1,100*F BY ROOM TEMPERATURE TENSILE PROPERTIES AS FOLLOWS, TENSILE AND YIELD STRENGTHS OF ABOUT 160-180 AND 135 145 KSI, RESPECTIVELY, TENSILE ELONGATIONS OF ABOUT 17-20%,, CHARPY IMPACT ENERGIES OF ABOUT 15-30 FT-LBS. AND HIGHER, AND ROCKWELL "C" HARDNESS OF ABOUT 35-40.
 2. Low, alloy, high strength, age hardenable, alloy steel consisting essentially in percentage by weight of about: 0.04% C max., 0.4-0.8% Mn, 0.2-0.35% Si, 0.5-1.5% Ni, 1.5-4.5% Cr, 1.5-3.5% Cu, 0.02-0.3% Nb, 0-0.12% Mo, 0-0.15% Al, 0-0.25% Ti, 0-0.005% B, 0.015% P max., 0.02% S max., balance iron except for impurities within commercial tolerances, characterized as hot rolled at about 2,250*-1,850*F and air cooled, and also as thereafter solution treated at about 1,600*-2,200*F and cooled at least as rapidly as air cool, by a Rockwell ''''C'''' hardness of about 30 and under, suitable for forming and machining, and as thereafter aged at about 700*-1,110*F by room temperature tensile properties as follows, tensile and yield strengths of about 160-180 and 135-145 ksi, respectively, tensile elongations of about 17-20%, Charpy impact energies of about 15-30 ft-lbs. and higher, and Rockwell ''''C'''' hardnessEs of about 35-40.
 3. A wrought article made of a steel according to claim 1 in the condition as hot rolled and air cooled and characterized by a hardness of about 30 Rockwell ''''C'''' and under, adapting the same for forming and machining, and therafter age hardening.
 4. A wrought article made of a steel according to claim 1 in the condition as solution treated and characterized by a hardness of about 30 Rockwell ''''C'''' and under, adapting the same for forming and machinine, and thereafter age hardening.
 5. A wrought article composed of an age hardened steel according to claim 1 having room temperature tensile properties as follows; tensile and yield strengths about 160-180 and 135-145 ksi, respectively, tensile elongation about 17-20%, Charpy impact energy about 15-30 ft.-lbs., and Rockwell ''''C'''' hardness about 35-40. 